This invention relates generally to electromagnetic solenoids intended to operate immersed in a fluid containing debris or magnetic particles, and more particularly to an improved low cost DC solenoid suitable for use immersed in oil or other liquids.
One use of a immersed solenoid is to shut off the flow of fuel in a diesel type internal combustion engine. The most effective location for the solenoid is inside the fuel injection pump. Locating the solenoid inside simplifies the design of the pump. However, when the solenoid is located inside the pump, it operates directly in the diesel fuel. Because of the magnetic field associated with a DC solenoid, ferrous particles in the fuel oil are attracted to the surfaces of the solenoid by the magnetic field. The contamination often builds up and mechanically prohibits the electric shut off solenoid from either opening or closing properly.
Solenoids designed to operate immersed in a fluid such as diesel oil containing debris or magnetic particles are also suseptible to loss of efficiency due to migration of debris or magnetic particles into the clearance space between the movable armature and core of the solenoid. The problem is particularly acute with a DC solenoid, since magnetic particles tend to accumulate and build up in the clearance spaces.
One prior art approach was to employ a smooth cylindrical armature sliding with close clearances within a smooth brass sleeve inside the bobbin, and to provide a sliding felt washer seal around the armature plunger in order to prevent entry of the debris or magnetic particles. While this has been partially successful, the felt washer and brass sleeve represent additional cost in material and labor, loss of efficiency because of friction with the armature, loss of sealing over the life of the solenoid because of wear of the washer, and a possibility of malfunction of the solenoid due to jamming action of the felt washer with the armature.
Solenoids are known of the type which operate fluid valves and permit the controlled fluid to flow through longitudinal grooves around the magnetic armature, such constructions being shown in U.S. Pat. No. 1,504,773 to Marston, 2,607,368 to Mayer, and 2,614,584 to Geopfrich. This type of solenoid requires a pressurized flow of fluid from one end of the solenoid armature to the other in order to conduct the fluid to the valve opening. In U.S. Pat. No. 4,518,938 to Bartholomaus et al., an armature with external longitudinal grooves slides within a low friction sleeve having a synthetic plastic material coating. Flow through the longitudinal grooves in the armature is due to pressure difference across the armature.
It would be desirable to provide a solenoid designed for fluid immersion which is tolerant to magnetic particles and other debris in the fluid.
Accordingly, one object of the present invention is to provide an improved low cost solenoid designed for operation immersed in a fluid containing contaminating debris and magnetic particles.
Another object of the invention is to provide an improved low cost solenoid for use inside a diesel fuel injection pump.